1. Field
This disclosure relates generally to class D audio amplifiers, and more specifically, to a system for generating a discrete noise-shaped variable switching frequency signal.
2. Related Art
In class D audio amplifiers, digital pulse width modulation (“PWM”) may be used to convert a digital signal into a series of pulses based on a switching frequency. The switching frequency may be quantized by a high frequency system clock. In conventional PWM, the switching frequency is fixed, which produces discrete tones in the signal spectrum located at the switching frequency and its harmonics. These discrete tones are undesirable since they may produce electromagnetic interference (“EMI”). In communication applications, the discrete tones may exceed EMI regulations. In class D audio amplifier applications, the discrete tones are more harmful than broadband noise since human perception is more sensitive to discrete tones.
Various methods have been used to eliminate or reduce EMI. For example, low pass filters have been used to eliminate EMI; however, low pass filters may require additional board space and expense. Frequency dithering has also been used to reduce EMI; however, frequency dithering is more suitable for analog PWM, since the switching frequency can be varied in a continuous fashion. In digital PWM, the switching frequency is based on counting the system clock; therefore, by definition, the possible switching frequencies are discrete and tonal. Moreover, in class D audio amplifier applications, the high frequency system clock may be generated from a high accuracy low noise crystal reference, and performing frequency dithering on the reference clock may produce noise in the audio band.